Historical Articles

Clear Protective Coatings for Copper-Chromium
Plate

OF THE PROBLEMS created
by our program of military preparedness the one posed by the prohibition of
the use of nickel for most civilian purposes was only a minor dislocation in
terms of the National economy. To the electroplater, however, it was a major
catastrophe. It meant that a chromium finish could no longer be produced by
plating, consecutively, copper, nickel and chromium. And the electroplater knew
that when chromium is plated directly on copper the resulting finish is sadly
deficient in protective ability. The automobile manufacturer also knew this
and was gravely concerned about the plating on the zinc-base die castings used
so lavishly for the ornamentation of automobiles.

Among all of the types of
plate, only nickel-chromium or copper-nickel-chromium combine to a satisfactory
degree adequate protection and the appearance that the public prefers. The only
apparent method of preserving the appearance of the chromium plate and at least
partially compensating for the protection lost by the elimination of nickel
undercoating was to apply a clear organic coating;

Up to that time organic
coatings for use on chromium plate had been mainly enamel products, and their
purpose had been decoration of such small parts as emblems, radiator ornaments
and door handles. Service requirements had not been severe. The main problem
had been to secure adhesion to the very smooth, chemically almost inert surface,
and this had been solved satisfactorily by minor modification of conventional
baking-enamel formulas based on alkydresin-amine formaldehyde-resin vehicles.

REQUIREMENTS
A clear coating for protection was an entirely different matter. The electroplater
specified that this coating must: (13 bake at a temperature not exceeding
300° F (149° C), preferably 275° F (135° C), to avoid blistering
of the plate on zinc-base diecastings, (2) have excellent adhesion, (3) have
abrasion resistance of a high order, (4) show no discoloration or dulling of
the bright finish, (5) impart good salt-fog resistance to copper-chromium plate,
and (6) have satisfactory weathering properties. Also mentioned were resistance
to water immersion, humidity, grease, and perspiration. Inasmuch as this sounded
like a rather large order, coating manufacturers were quick to realize that
it called for evaluation of all of the resins and resin combinations that might
conceivably prove advantageous, with emphasis on the newer developments that
had not been thoroughly tested in chromium coatings.

Phenol-formaldehyde resins
were ruled out by their tendency to discolor.

In most cases each sub-group
was represented by more than one specific resin, amino-formaldehyde resins by
six. With one exception, each formula contained two or more resins. A majority
of the formulas contained only two resins, which differed in type. Thirty separate
formulas were included in one series of preliminary, or screening, tests by
one coating manufacturer.

In this series of screening
tests, the coatings were applied on copper-chromium plated test panels supplied
by the Doehler-Jarvis Corporation and cleaned by it. The coatings were sprayed
to a dry-film thickness of approximately 1 mil (25 ), and baked 20 minutes in
a convection oven at an air temperature of 300° F (149° C). The films
were tested only for hardness, toughness, and adhesion, with a view to elimination
of all materials that were deficient in these properties. Only three materials
showed promise: (1) a combination of alkyd and amino resin, (2) a combination
of Epon ester and amino resin, (3) a combination of methacrylate resin and nitrocellulose.
The latter was dropped from further consideration because of its low solids
content at spraying consistency and the extremely thin dry film produced. Thus,
two types of formulas remained for more complete evaluation.

In the above-mentioned screening
tests the baking had been done on a single schedule, which had been chosen somewhat
arbitrarily. At this point it was decided to determine the optimum baking schedule
before conducting further evaluation of the materials. The alkyd-amino resin
formula that had given best results was chosen for this purpose. It was necessary
to establish some method and standard for determination of satisfactory conversion,
or cure. By correlating salt-fog results with measurements of film hardness,
it was found that a hardness of H by the pencil method was adequate for excellent
performance. (H hardness means that the coating cannot be scratched with an
H pencil but can be scratched with a 2H pencil.) The coating was sprayed on
heavy test panels to a dry-film thickness of approximately 1 mil (25 µ)
and baked in an electric convection oven at six different air temperatures ranging
from 275 to 400° F (135 to 204° C) until a hardness of H was obtained.
The resulting optimum baking schedules are found in Fig. 1. Inasmuch as a baking
time of 40 minutes is excessive, and temperatures above 300° F (149°
C) are likely to cause some blistering of plate on zinc-base alloy, it was decided
to adopt a schedule of 30 minutes at 300° F (149° C) for the remainder
of the testing program.

The two formulas that had
survived the screening tests were then subjected to the following tests
1. Water immersion at 100° F (38° C) for a minimum of 168 hours, with
the films being scored to the plate
2. Salt fog at 95° F (35° C), using a 5 per cent solution, for a minimum
of 168 hours. The films were again scored to the plate
3. Weather-O-Meter test for 250 hours
4. Exposure to intense ultra-violet light for 24 hours
5. Exposure to 50 per cent acetic acid for 4 hours, to obtain an indication
of resistance to perspiration
6. Impact test for adhesion
7. Abrasion resistance by the Tabor Abraser, using CS 10 wheels and 1000-g load

In the foregoing tests the
alkyd-amino resin formula proved definitely superior to the Epon ester-amino
resin formula in color retention under ultra-violet light, resistance to abrasion,
and resistance to salt fog, and equal in all other properties. It met the
most severe specifications of automobile manufacturers. Against a salt-fog requirement
of 168 hours, it showed no creepage from the score line or other failure after
250 hours. In a later test it was subjected to five cycles of the following:

After the five cycles, the
finish was subjected to salt fog for 250 hours, and there was no failure.

This description of how
coating manufacturers solve a problem may have created the impression that the
final answer is simply to mix an alkyd resin and an amino resin. Any such impression
needs be corrected. Alkyd resins come in a very wide range of compositions and
properties, and it is important to use the particular alkyd resin or mixture
of alkyds that is best for a clear coating over chromium. In like manner, the
amino resin requires careful selection for best performance in the particular
formula. Finally, the proportion of alkyd resin to amino resin must be closely
adjusted to give optimum results.

PERFORMANCE FACTORS
It is safe to say that no electroplater has been able to obtain consistently
the best performance that is inherent in the coatings and reflected by the laboratory
test results that have been described. The use of these coatings certainly raises
many problems, and the results are governed in large measure by the attention
and understanding which are applied to these problems.

Treatment of the Chromium
The first factor that calls for consideration is the cleaning and conditioning
of the parts before the clear coating is applied. Occasionally chromium plate
requires buffing, and buffing compounds may contain wax. A recent analysis of
three kinds of buffing sticks that were represented as wax-free disclosed that
all three contained wax. An alkali wash at 185° F (85° C) is reported
to be effective. The temperature is important; it should be higher than the
melting point of the wax. Following the alkali wash and a cold-water rinse to
remove excess alkali, the prevailing practice is to rinse with very dilute chromic
acid and finally with deionized water. The clear coating should be applied
as soon as feasible after the prepaint treatment, preferably on the same day,
as contamination from the air may reduce the adhesion and affect other properties.

Coating Thickness
Next in order is the application of the coating. The main point here is that
it should be applied uniformly and in adequate film thickness. As a rule dipping
is eliminated by the shape of the parts, leaving spraying as the only feasible
method of application. The difficulty of properly controlling the film thickness
is aggravated by the variable shapes of the parts and by the transparent nature
of the coating, which makes it difficult for a hand-spray operator to judge
the thickness being applied. Parts from production lines have shown a dry film
thickness as low as 0.2 mil (5 µ) and as high as 3 mils (76 µ).
The only means for proper control seems to be automatic spraying. Uniformity
of film thickness over the entire surface of the individual parts can be improved
further by electrostatic spraying with suitable equipment. Another advantage
of electrostatic spraying is that it offers possibilities of reducing the loss
by overspray, which at best is large, by 50 percent or more. Both the automatic
sprayer and the electrostatic equipment must be carefully chosen and engineered
to fit the job. One automobile manufacturer specifies a dry film thickness of
1± 0.1 mil (25 ± 2.5 µ). This is a fine goal, but the electroplater
will be doing well if he maintains a range of 0.7 to 1.2 mil (18 µ to
30), which will insure good performance. There is little chance of controlling
the film thickness without frequent determinations, and this calls for both
a wet-film thickness gage and a dry-film thickness gage. These instruments will
pay for themselves many times over by reduction in the number of rejects.

Baking Cycle
Care n metal preparation and control of film thickness will both be wasted unless
there is equal attention to the baking cycle. The coatings that have proved
best for protecting chromium plate are at least in part of the heat-convertible
type. This means that their good properties are not developed unless they are
baked in a manner that will accomplish the conversion. A partially cured
coating is likely to exhibit only a small fraction of the protection of the
same coating when adequately cured. It is obvious that a thin coating on metal
will be substantially the same temperature as the metal and, therefore, that
the metal temperature controls the rate of cure. When heating is by convection
there is a considerable lag between the air temperature and the metal temperature,
the amount of lag depending on the thickness of parts and the rate of air circulation.
When heating is by radiation instead of by convection, the metal temperature
normally rises faster than the air temperature. Some of the newer ovens combine
the convection and radiation principles of heat transfer in various degrees.
These few facts should make it obvious that it is impossible to specify the
cure in terms of air temperature and length of time. It could be specified as
a given metal temperature for ;a given time period, but the measurement of metal
temperature with a thermocouple is hardly feasible for daily control of production.
This brings us back to the point that our real object is a certain degree of
cure, which can be determined by measuring the hardness of the film, most feasibly
by the pencil method. It has been found that a hardness of H assures good performance
of typical clear coatings on chromium. When a convection oven is used it is
only necessary (1) to determine the air temperature and time required in the
particular oven to produce a minimum hardness of H and (2) to maintain these
conditions. When baking is by infra-red the arrangement of lamps and the conveyor
speed should be adjusted so that a hardness of H be obtained. In either case
the hardness should be checked at frequent intervals. As a further check on
conveyorized convection ovens, a recording thermometer may be sent through at
intervals. A typical satisfactory schedule for convection ovens is 30 minutes
at 300° F (149° C). This temperature is safe for zinc-base diecastings
provided the castings are dense and sound and the plating process has been carried
out properly. If these conditions have not been met, it may be necessary to
drop the temperature to 275° F (135° C) in order to avoid blistering
of the plate.

STRIPPING
In even the best regulated plants, a small proportion of the finished work will
be rejected. This creates the problem of stripping. When fully heat cured the
better clear coatings for chromium now in use are remarkably resistant to strippers.
In fact, when these coatings made their advent, none of the strippers then available
were satisfactory for the purpose. After months of work and several field trials
some stripper manufacturers succeeded in developing effective materials. It
is now possible to strip in as little as 5 minutes by immersion in a stripper
solution at 200° F (93° C). When strippers are evaluated consideration
should be given to the physical condition in the stripping bath of the coating
that has been removed. Some strippers leave the coating as small particles that
tend to cling to the stripped parts; others cause the coating to form into balls
and permit the parts to come out of the bath clean.

While creating a stripper
problem, the use of clear coatings for chromium solved another problem, that
of rack coatings. During the spraying of the parts on a rack, the rack is also
coated completely. This coating when baked withstands both the cleaning baths
and the plating baths. Normally the racks receive a new coating during each
complete cycle, and no other rack coating is required. Racks that have been
in production for many months have required no attention to the coating. The
flaking or powdering in the cleaning cycle has been sufficient to prevent excessive
build up.

COATING REPAIR
By comparison with straight metallic coatings, the greatness weakness of organic
coatings is that they are much more susceptible to becoming scraped off by the
accidental bumping and rubbing that may occur in service. When this happens
the local damage needs to be repaired. It is obvious that the coating for this
purpose should be suitable for application by brushing or spraying and should
not require baking. Coatings of this type have been developed and are now available
through automotive retail outlets. In nature they are synthetic varnishes. In
performance they fall considerably short of the baked coatings, as would be
expected of the air-drying or oxygen-convertible type.

COATINGS ON ZINC
The baked coatings that were developed- for chromium have also been tested on
buffed zinc-base diecastings and on zinc plated zinc-base diecastings. It is
well known that zinc is chemically more reactive than chromium, and hence
its need for protection from the atmosphere is even greater. On both buffed
zinc alloy and zinc plate the coatings showed good adhesion and good resistance
to salt spray. It is understood that zinc requires a different prepaint conditioning
than chromium. The time between conditioning and coating should be as short
as possible, preferably no longer than two hours, if good adhesion is to be
obtained. Should the use of chromium coatings be prohibited for most civilian
purposes, zinc with baked organic coatings can take their place at least in
some cases.

Much of the credit for the
clear coatings that are being used today on chromium goes to the electroplaters
who gave unstinted cooperation to its development, along with constant and vigorous
prodding. The work of the coating chemist is never finished, although he cannot
claim that he never dies. By continued cooperation the plater can look forward
with assurance to still better coatings in the future.

ACKNOWLEDGMENT
The author expresses appreciation of helpful information and criticisms from
Dennis 17. Roelofs, of the Grand Rapids Varnish Corporation, and Lyman B. Sperry
and Edward W. Gross, of the Doehler-Jarvis corporation.

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